A backlight unit, forming a backlight device of a liquid crystal display, is mainly composed of light sources and a light guide plate for guiding light rays emitted from the light sources to radiate the light rays with in-plane light radiation. The light rays emitted by the light sources are guided from a lateral surface into the bulk of the light guide plate and emitted from one of the major surfaces of the light guide plate by in-plane light radiation to illuminate a liquid crystal display panel. The illuminating light rays, emitted by in-plane light radiation from the light guide plate, are the light rays transmitted through a light diffusing sheet or a lens sheet, provided to the backlight unit, so that uniform in-plane light radiation may be achieved on the entire surface of the liquid crystal display panel.
As the light sources for the backlight unit, fluorescent tubes or light emitting diodes (LEDs), for example, are used. In particular, the light emitting diodes are used as light sources mounted to electronic equipment which are small-sized and of a thin thickness. For example, in a backlight device of a liquid crystal display panel, with a size on the order of several inches, mounted on a portable small-sized electronic apparatus, such as a mobile phone, PDA (Personal Digital Assistant) or a digital camera, chip type white light emitting diodes, are used as light sources.
In a backlight device for illuminating a liquid crystal display panel of a large format, such as a display for a PC (Personal Computer) or for a television receiver, phosphorescent lamps, such as CCFLs (Cold Cathode Fluorescent Lamps), are used as light sources.
The CCFL (Cold Cathode Fluorescent Lamp) suffers a problem that it is inferior in power consumption or useful life to the LED, or that it may have undesirable influences on global environments due to use of mercury as a gas sealed therein.
Hence, there has been made a proposal which resides in using light emitting diodes as light sources for a backlight unit illuminating a liquid crystal display of a large format, such as a display for a PC or a television receiver. The light sources for the backlight must generate white light. However, the white color emitting diodes, used as light sources for the backlight unit, illuminating the liquid crystal panel of a small format, are designed to yield white light by a phosphorescent material coated on the blue light emitting diode. These white color emitting diodes are inferior to the CCFLs in the light emitting efficiency by approximately ⅙ pr 1/10, and hence are difficult to use for a liquid crystal display of the large format.
Consequently, there has also been made a proposal which resides in using light emitting diodes, respectively emitting three prime colors of light, namely red, green and blue light rays, and in mixing these red, green and blue colors, emitted from the light emitting diodes, to yield the white color light. By using three light emitting diodes for yielding the white light, sufficient luminance may be assured, while deterioration in the light emitting efficiency may be lower than with the aforementioned white light emitting diodes.
Referring to FIG. 19, a backlight unit 110, employing light emitting diodes 111R, 111G and 111B, emitting red, green and blue light rays, respectively, as light sources, and which is configured for illuminating a transmission type liquid crystal display panel 120 with in-plane light radiation, will now be explained. It is noted that, if the light emitting diodes 111R, 111G and 111B need not be distinguished from one another, they are collectively referred to as light emitting diodes 111.
The backlight unit 110 includes the light emitting diodes 111R, 111G and 111B, as light sources, a light guide plate 112, guiding the light emitted by the light sources, a light diffusing sheet 113, a first lens sheet 114 and a second lens sheet 115. The light diffusing sheet and the first and second lens sheets are layered in this order sequentially on a light radiating surface of the light guide plate 112.
In FIG. 19, only one each of the light emitting diodes 111R, 111G and 111B is shown. However, in actuality, there are provided a number of the sets of light emitting diodes, which is matched to the size of the liquid crystal display panel 120 adapted for in-plane light emission and illumination.
The light emitting diodes 111R, 111G and 111B, as light sources of the backlight unit 110, emit red light rays Lr, green light rays Lg and blue light rays Lb, respectively. The red light rays Lr, green light rays Lg and blue light rays Lb, emitted by the light emitting diodes 111R, 111G and 111B, respectively, undergo spontaneous color mixing by passing through a light guide path 116 and a light reflecting path 117 to then fall on the light guide plate 112 as white light. The light guide path 116 and the light reflecting path 117 are provided for affording a spacing necessary for the red light rays Lr, green light rays Lg and blue light rays Lb, emitted by the light emitting diodes 111R, 111G and 111B, respectively, to be mixed in color by spontaneous color mixing.
FIG. 20 depicts a cross-section taken along line A-A shown in FIG. 19. Referring to FIG. 20, the spacing necessary for the red light rays Lr, green light rays Lg and blue light rays Lb to be mixed in color by spontaneous color mixing is prescribed by adequately setting the width of the light guide path 116 and/or the radius R of the light reflecting path 117. The light guide path 116 and the light reflecting path 117 are formed of a material having a refractive index necessary for efficiently guiding the incident light rays to the light guide plate 112.
The white light, incident on the light guide plate 112, is guided through the bulk of the light guide plate 112 as it undergoes total reflection therein. On a light reflecting surface 112b of the light guide plate 112, there are formed prism patterns and/or dot patterns for efficiently changing the proceeding direction of the incident light to a direction towards an inner light radiating surface 112c. The light rays incident on the inner light radiating surface 112c with an angle of incidence less than a critical angle of incidence are emitted by these patterns from the inner light radiating surface 112c. 
The light rays, emitted from the light radiating surface 112c, suffer from significant variations in the in-plane light volume distribution. Consequently, these light rays are caused to be incident on the light diffusing sheet 113 in order to be equalized in the in-plane light volume distribution. The light rays, radiated from the light diffusing sheet 113, are incident on the first lens sheet 114 and the second lens sheet 115 and thereby deflected to proceed in a direction corresponding to the direction of a normal line to the light radiating surface 112c. 
By causing the light rays, emitted by the light radiating surface 112c of the light guide plate 112 and traversing the light diffusing sheet 113, to pass through the first lens sheet 114 and the second lens sheet 115, the front side luminance of the backlight unit 110 may be improved efficiently.
Meanwhile, a backlight device, having light emitting diodes, emitting red, green and blue light rays, as light sources, and which is different than the backlight unit 110 explained with reference to FIGS. 19 and 20, has been described in each of the JP Utility Model Publication Hei7-36347 and JP Patent Publication Kohyo 2002-540458.
In the backlight unit 110, shown in FIGS. 19 and 20, the light guide path 116 and the light reflecting path 117 are provided in such a manner as to increase the thickness of the light guide plate 112, in order to provide for spontaneous color mixing of red, green and blue colors, emitted from the light emitting diodes 111.
Thus, if the backlight unit 110 is mounted to the liquid crystal display panel 120 to form the liquid crystal display, the display is increased appreciably in thickness.
In case the illuminating device uses light emitting diodes, emitting light rays of three prime colors, that is, red light rays, green light rays and blue light rays, as light sources for the backlight unit illuminating a liquid crystal display panel of a large format, and the three colors are mixed together to form white light, it is necessary to increase the number of the illuminating devices in use in order to provide for desired luminance in keeping with the increase in size of the liquid crystal display panel. Hence, there is raised a demand for lowering the cost of each illuminating device and for elevating the light utilization efficiency.
Concomitantly, a there is raised a demand for willfully using an inexpensive light emitting diode, exhibiting variations in characteristics, instead of an expensive light emitting diode. However, if such inexpensive light emitting diode is used, it is not possible with the state-of-the-art illuminating device to produce the white light of high color purity.